During the printing of long jobs, especially those with color critical output, it is often desirable to view sample copies. Viewing samples allows the press operator to ensure that the appropriate content is being printed and that the appearance of the output matches the customers expectations. When offset presses are used, examining a sample is fairly straightforward. Copies of just one page are being produced at any one time and there are access points in the printing press that allow sheets to be viewed, and in some cases retrieved, during the printing process.
An offset printing press might, for example, have five print units. Each print unit applies one color in a multi-color printing operation. The press is usually open and the quality of the print job can be monitored by looking at the output of each print unit as it is conveyed from one print unit to the next. Adjustments to the print units can be made as the job runs.
In digital production printing, multi-page documents are being produced and delivered via closed paper paths to finishing devices that do not allow convenient access to printed output until the printing process is complete. As a result, it is extremely difficult to monitor product quality during a digital production printing job. If a problem develops in the middle of a job it is not noticed until after the job is complete. In large jobs, this can be very costly, both in wasted time and material.
Some attempts have been made to give digital printing systems more of the on-the-fly adjustability that is common in offset presses. U.S. Pat. No. 5,606,395 to Yang et al. and assigned to Xerox Corporation, is for a method and apparatus that allow an operator to monitor a digital printing system's output tray and enter fine-tuning commands through an electronic control. The operator can determine the effectiveness of his commands by watching how they affect the sheets that arrive at the output tray.
This technique is most effective when copies of only one page are being printed. When collated copies of multi-page documents are being produced, and the sheet or sheets of interest are covered by the top sheet, this technique loses its usefulness. In order to check the adjustments, the operator has to remove a completed compilation from the output tray, unwrap it (if it is shrink wrapped), and sort through the sheets to find the sheet or sheets of interest. If further corrections are needed then all the copies created in the meantime, have to be discarded or reworked. In either case, time and material are wasted.
An improvement is needed in digital production printing systems that allows print quality to be monitored in real-time.
The present invention is such an improvement. It allows a user to specify which parts of a job are critical, or representative of the job. The user can also indicate how often sample copies of each of those parts should be produced. The samples can be assessed, either by the user or a device. If a problem develops in the middle of a run, such as, poor registration, streaking or color drift, the operator will see it reflected in the samples and can pause the job and take corrective action and then resume the job. Alternatively a sensor can examine the samples and provide information to the machine to allow it to make automatic corrections to the process.